Natural oil-based polyols (NOBP) are typically polyester or hybrid polyether/polyester polyols, and are derived at least partially from plant-sourced triglycerides (oils). These oils consist of mostly C18 fatty acids containing from 0 to 3 sites of unsaturation each. In order to manufacture polyols, the fatty acids are isolated away from the glycerin core and are functionalized to provide an alcohol/methyl ester monomer in a process which has been well-described, e.g., U.S. Pat. No. 7,615,658. The hydroxymethyl functional groups are added via a hydroformylation reaction followed by hydrogenation which reduces aldehydes to alcohols and unsaturated carbon bonds to saturated hydrocarbons.
In the final step of the natural oil polyol production, the hydroxymethylated methyl esters are reacted with an initiator to build the finished polyol product. This transesterification reaction is run in the presence of a Lewis acid catalyst, e.g., stannous octanoate, which serves as the catalyst. During polymerization organic chromophores are formed in the polyol. While these types of reactions are rare due to the dilute nature of the precursors, they can produce undesirable color in the polyol.
APHA color is a color standard named for the American Public Health Association and defined by ASTM D1209. One color specification for polyol products is 40 APHA with some end-use applications requiring even lower color (e.g., less than (<) 20 APHA). Some natural oil-based polyols have a color of greater than (>) 40 APHA upon completion of the polymerization step.